1. FIELD OF THE INVENTION
This invention relates to a reversible light-sensitive glass having a refractive index of 1.5 to 1.55, in which the percent transmission of the glass changes abruptly and reversibly due to irradiation with ultraviolet light and visible light of short wavelengths.
2. DESCRIPTION OF THE PRIOR ART
Generally, it is a matter of course that using reversible light-sensitive glasses as light-shielding eyeglass lenses, those light-sensitive glasses having fast light-response are desired. If the refractive index of the glass can be increased, the thickness of the eyeglass lenses can be reduced. Furthermore, if the refractive index is adjusted to 1.523, polishing devices and measuring devices for eyeglass lenses in general can be used without modification.
However, it has been difficult to adjust the refractive index to this value without reducing the light response, especially the rate of fading, and also with a stable composition in which the precipitation of crystals other than silver halide crystals does not occur.
A reversible light-sensitive glass having superior light response as disclosed in U.S. patent application Ser. No. 232,667, filed Mar. 8, 1972, has already been provided. This glass was prepared as a result of finding the effect of the co-presence of BaO and K.sub.2 O in the base glass, and when the amount of BaO is restricted in order to maximize the light response and the rate of fading, it is impossible to increase the refractive index (n.sub.d) to more than 1.50.
A reversible light-sensitive glass having a refractive index (n.sub.d) of at least 1.50 and having superior light response as described in U.S. patent application Ser. No. 457,304, filed Apr. 2, 1974, has also been provided. This glass has an increased refractive index as a result of incorporating TiO.sub.2, ZrO.sub.2 and Nb.sub.2 O.sub.5 into the glass of the above described U.S. patent application Ser. No. 232,667, filed Mar. 8, 1972, now U.S. Pat. No. 3,833,511. In order to prevent strong coloration caused by the introduction of TiO.sub.2, it was necessary to incorporate a small amount of As.sub.2 O.sub.3 or Sb.sub.2 O.sub.3. However, it is difficult to obtain a glass having stable light responding properties by the introduction of As.sub.2 O.sub.3 or Sb.sub.2 0.sub.3. Furthermore, in the above application, a characteristic feature resides in a composition having a relatively larger proportion of Al.sub.2 O.sub.3, but the introduction of ZrO.sub.2 into such a composition brings about an increase in the liquidus temperature of the glass and thus a reduction in its thermal stability. This results in the precipitation in the glass of crystals other than silver halide crystals, and makes it difficult to obtain a reversible light-sensitive glass having excellent transparency.
In the reversible light-sensitive glasses now commercially available, the proportion of BaO is increased and also PbO, which adversely affects the light response and the rate of fading, is introduced in order to adjust the refractive index to 1.523. As a result, the resulting glass has a slow light response, especially a slow rate of fading, and more than 60 minutes is required for 90% of the coloration to disappear.
Furthermore, many of the reversible light-sensitive glasses of this kind do not have an entirely satisfactory weatherability which is required especially for glasses which tend to be exposed directly to the outer atmosphere.